A hybrid electric vehicle may perform regenerative braking operations during vehicle deceleration in order to capture and store energy that would otherwise be lost as a result of performing mechanical braking operations. In particular, during deceleration an electric energy conversion device (e.g. generator) may absorb kinetic energy generated as a result of the vehicle deceleration and may convert the kinetic energy to electric energy which may be used to increase the state of charge of a battery. Performing regenerative braking operations may be desirable beyond recouping energy because the hybrid electric vehicle may be decelerated with minimal or no negative torque supplied by the mechanical wheel brakes. Thus, reducing wear on the mechanical wheel brakes. During vehicle operation, regenerative braking may be in favor of mechanical braking via wheel brakes due to the battery being charged to capacity. Regenerative braking may be resumed when electric energy has been depleted from the battery so that the battery has room to store more electric energy. The incessant switching between regenerative braking and mechanical braking may be sensed by the vehicle operator and the vehicle operator may perceive the switching to be objectionable. Further, additional use of the mechanical wheels brakes may cause additional wear over time.
In one approach, regenerative braking operations may be extended by distributing excess electric energy to various vehicle systems for heating purposes. In particular, regenerative braking may be performed for a period after a battery has been charged to capacity by directing energy recouped from regenerative braking to a resistive heater in the coolant circuit of the internal combustion engine to heat the engine in order to improve emissions. Further, in a hybrid vehicle system, such as described in U.S. Pat. No. 6,574,535 for example, excess electric energy may be supplied to heat the passenger compartment heaters of the hybrid electric vehicle.
However, the inventors herein have recognized some issues relating to the above approach. Specifically, in some conditions, regenerative braking operations may be suspended after a period in order to prevent the engine coolant from overheating the engine or to prevent the vehicle cabin from becoming uncomfortably hot. Thus, although the above approach may extend regenerative braking capabilities for a period, eventually mechanical wheel brakes may be used earlier than desired to decelerate the vehicle
In at least one approach described herein, at least some of the above issues may be addressed by a method of generating brake force to decelerate a vehicle comprising an internal combustion engine coupled to an input of a first electric energy device, the first electric energy device having an output coupled to a transmission device, and a second electric energy conversion device coupled downstream of the transmission device, the first and second electric energy conversion devices being powered at least by a battery, the method comprising: during a deceleration condition, generating brake torque via the second electric energy conversion device to generate electric energy storable in the battery while rotating the internal combustion engine via torque output from the first electric energy device to deplete electric energy from the battery.
In this way, regenerative braking may be performed to decelerate the hybrid vehicle even for an extended period to further reduce usage of mechanical wheel braking operations. As a non-limiting example, regenerative braking operations may be performed for longer durations since once the battery is charged to capacity or a selected level, the first electric energy conversion device may be adjusted to deplete an amount of electric energy that tracks the amount of electric energy being created by the second electric energy conversion device in what may be considered a charge neutral state. Further, the amount of electric energy depleted may be adjusted based on the amount of electric energy generated to control the state of charge to a desired level during the braking operation.